The present invention relates to packet-based Orthogonal Frequency Division Multiplexing (OFDM) in wireless digital communication systems. More specifically, the present invention relates to a system and a method for frame and frequency synchronization in packet-based OFDM.
Advancements in communication technology have resulted in parallel transmission schemes for transmitting data at very high rates over time-dispersive radio channels. One such scheme for parallel transmission is packet-based OFDM, which is a transmission technique based on the concept of Frequency Division Multiplexing (FDM). In FDM, multiple signals are transmitted simultaneously over a single transmission path, but on different frequency bands. Extending this concept further, the different carriers used for transmitting multiple signals over different frequency bands are orthogonal to each other in OFDM. As a result, an OFDM bandpass signal is the sum of a number of orthogonal sub-carriers, with the data on each sub-carrier being modulated independently. The sub-carriers can be modulated by using various modulation techniques, for example, Quadrature Amplitude Modulation (QAM) and Phase Shift Keying (PSK). The OFDM provides high spectrum efficiency, resistance against multipath interference, and facilitates a simple channel equalizer.
A significant challenge associated with packet-based OFDM is frame synchronization and frequency-offset estimation. Frame synchronization in OFDM refers to frame alignment of the received stream of data. This is achieved by exploiting distinctive bit sequences. Subsequently, data bits within the frame can be extracted for decoding and retransmission. Further, frequency-offset represents a frequency difference between the carrier signals generated by oscillators of a transceiver. Frequency-offset estimation refers to the precise estimation of the frequency difference. Frequency-offset estimation enables frequency synchronization in packet-based OFDM.
Several methods have been conventionally used for frame synchronization and frequency-offset estimation in packet-based OFDM. Schimdl and Cox (Robust frequency and timing synchronization for OFDM, IEEE Trans. Commun., vol. COM-45, pp. 1613-1621, 1997) proposed one such method for frame and frequency synchronization in packet-based OFDM that uses a preamble of certain structure. According to this method, a timing metric is used, which exploits the symmetry in the first symbol of the preamble. The first symbol consists of two identical halves in the time domain. The frequency-offset within a ±1 subcarrier spacing is estimated from the phase of the numerator term of the timing metric at the boundary of the first symbol of the preamble. Moreover, frequency-offset above ±1 subcarrier spacing is estimated using the second symbol of the preamble. However, the proposed timing metric suffers from a plateau, which causes ambiguity in determining the start of the frame. As a result, frame synchronization may not be accurate.
In light of the foregoing discussion, a need exists for a method and system for frame synchronization and frequency-offset estimation, in an accurate and unambiguous manner, for packet-based OFDM. The present invention addresses such a need.